FIG. 1 is a one-line diagram of a typical arrangement of a main ac supply (e.g., power grid) 10, an on-line UPS 12, a load 14 and a battery bank 16. The on-line UPS typically contains a controlled rectifier DR.sub.1, a dc-to-ac inverter A.sub.1, a static bypass switch S.sub.1, and isolation transformers T.sub.1 and T.sub.2 (T.sub.2 is typically a nine-winding transformer, with a three-phase primary for S.sub.1, a three-phase primary for A.sub.1 and a three-phase secondary for the load). The dc bus 12-1 is designed specifically to support the bank of batteries 16. The bus can provide charging current and also draw current from the batteries in the event of an outage on the main ac supply 10. UPSs usually contain a static (electronic) bypass switch S.sub.1 so that the output of inverter A.sub.1 can be switched to the main supply 10 in the event of a fault or high inrush load.
FIG. 2 depicts an example of a system employing an auxiliary generator 18 in combination with a UPS 12. Battery supplied UPSs provide backup power for short periods of time, typically on the order of minutes. The backup time can be substantially increased by adding a generator to the system. This is typically accomplished using an automatic transfer switch S.sub.2 at the input of the UPS 12, as shown in FIG. 2. When the main ac power supply 10 fails, the batteries 16 supply power to the UPS. If the power remains down for a predetermined period of time, the automatic transfer switch S.sub.2 will start the auxiliary generator 18 and switch the UPS 12 input to the auxiliary generator output through S.sub.2.
The solution shown in FIG. 2 has several drawbacks:
1. The controlled rectifier DR.sub.1 in the UPS 12 tends to draw harmonically rich currents that are not easily handled by the generator 18. It is therefore necessary to oversize the generator or to add substantial filtering in front of the UPS. Both of these solutions increase cost and losses. PA1 2. Operating the generator 18 in grid-parallel mode requires added complexity. Circuitry must be provided for synchronizing the generator with the grid (i.e., the mains 10). In most circumstances, parts must also be added to prevent the generator from generating power into the grid in the event of a power failure.
Accordingly, a goal of the present invention is to provide a combined UPS/auxiliary generator system that avoids the problems discussed above, and that provides improved compatibility with many forms of stored or generated energy.